1. Field of the Invention
The present invention relates to a method and apparatus for material inspection, and more particularly, to a method and apparatus for detecting defects in transparent or translucent material moving through a liquid bath.
2. Background of the Invention
Inspection is an essential part of any process for manufacturing materials. Product manufacturers must ensure that the feedstock they use to manufacture their products meets or exceeds standards of quality, such as size, color, and purity. Inferior feedstock degrades the quality of the final product and reduces the manufacturer's sales and profits. Thus, to maintain strict product specifications and satisfied customers, manufacturers demand that the feedstock adhere to a minimum quality or grade.
Sensitive to such concerns, suppliers of raw materials routinely conduct inspections with objectives such as identifying and removing flawed material, assessing the overall quality of a batch of material, and separating a batch of material into portions of like size, color, purity, or grade. In addition, manufacturers must be able to determine, as early as possible in the process, whether the manufacturing equipment is producing flawed material. This early determination allows the manufacturer to take timely corrective action.
In the plastics industry, for example, manufacturers often shut down the extruders that produce the raw materials for routine maintenance. This maintenance procedure ensures that the extruder is always producing optimum product. Unfortunately, this maintenance procedure can be very time-consuming. If the maintenance procedure is conducted too frequently, valuable production time can be unnecessarily lost to maintenance time. The extruder production time can be maximized by automatically detecting the onset of flawed material in the manufacturing process, rather than relying on scheduled maintenance that may, in fact, be too frequent or too late.
When inspecting material, suppliers and manufacturers look for a variety of defects, depending upon the type of material. In food products, for example, defects include foreign matter, uncooked portions, unprocessed or clumped portions, and contaminants from pests such as insects or rodents. In plastic pellets, defects generally include foreign matter, charred raw material, contaminants from unmelted base constituents of the polymer material (often referred to as gels), incorrectly sized or colored pellets, broken pellets, and pellets that are stuck to each other. In addition, manufacturers sometimes measure the amount of fines (small chips or thread-like pieces that can break away from the pellets during manufacturing and transportation).
Traditionally, material inspection has been a slow, labor-intensive process limited to testing small samples instead of all material that is incorporated into the final product. Thus, theoretically, a sample might not be representative of the defects present in the rest of the material. In addition, although manufacturers would prefer to inspect material as it is being produced, in many instances this practice is not possible.
Although the following discussion of the traditional methods of inspection is in the context of the plastics industry, the methods and their associated drawbacks apply equally to other material inspections, such as inspections conducted during food processing. In the plastics industry, the current methods for inspecting raw plastic material include: 1) visual inspection of pellets by a person; 2) inspection of polymer ribbons formed from pellets; 3) inspection of molten polymer; and 4) automated inspection of the pellets. It is important to note that these methods are typically only suitable for base or raw materials that are transparent or translucent. Generally, however, this requirement is not a problem because coloring is usually added late in the manufacturing process.
Visual inspection of pellet material by a person is the most common method of material inspection. It is generally conducted in a quality control laboratory separate from the manufacturing process. The visual inspection method typically involves spreading a sample of particles on top of a light table (e.g., a glass or Plexiglas™ table with a light source below its top) or other white or light-colored surface, and examining each particle for a defect. If the size of a possible defect is small, the inspectors must strain their eyes to observe the defect or perhaps use a magnifying glass to focus on each particle. Although using the light table or light-colored surface enhances the defects, the process is only as reliable as the eyes and concentration of the human inspector. In addition to human error, using human inspectors increases labor costs and significantly reduces the speed at which material is analyzed. Visual inspection is therefore inappropriate for the in-line inspection of all material used in a process.
Inspection of polymer ribbons involves melting a sample of raw material pellets into a molten form, extruding the molten material into thin, ribbon-like shapes, and inspecting the ribbons for defects. The ribbon shapes are flatter than pellets, which eases handling and presents a larger viewable surface area This ribbon inspection technique can be incorporated into manual (visual inspection by a person) en and automated methods of inspection. Despite the advantages in handling and viewable surface area, the ribbon inspection technique suffers from the added time and expense of melting the raw material pellets. The equipment and manpower needed to accomplish this extra step add significantly to the overall cost of material inspection. Moreover, the extra steps involved in melting and extruding the material make this technique suitable only for evaluating small samples of process material and incompatible with the in-line inspection of all process material.
In addition to analyzing ribbons, some inspection techniques analyze the molten polymer itself, in a device known as a flow cell. The flow cell is a chamber with a conduit viewable through a window. U.S. Pat. No. 4,910,403 to Kilhamn et al. discloses a flow cell typically used for the molten polymer inspection technique. The molten polymer is channeled through the conduit, illuminated, and inspected as it passes under the window. This inspection technique can analyze the molten material either manually or with an automated device.
Although flow cell inspection techniques can identify defective portions of molten polymer, the techniques cannot separate those defective portions from the remaining acceptable portions. Thus, the method is suitable for grading the molten polymer or monitoring a manufacturing process for quality control, but not for removing defective portions and improving the quality of the molten polymer. In addition, the flow cell and the equipment necessary to convey the molten polymer introduce additional costs and complexities to the inspection process, and, because of their limited processing speed, are suitable only for spot-checking material. Indeed, Kilham et al. describes the inspection of only a portion of the fill volume of a polymer being manufactured. Thus, conventional flow cell inspection techniques are not optimal for inspecting all of the material moving through an extruder,
While each of these methods offers a means to quantify defects in or grading polymer material through a sampling mechanism, none of these means offers a complete in-line inspection of all process materials.